The mini-PET system employed in this study incorporates two blocks of BOO
detector crystals. Each detector module contains a 25 x 54 mm block of BOO which
is segmented into a 6 x 8 array of crystal elements, 3.5 mm ( transaxially ) by 6.25
mm ( axially ) and 30 mm ( deep ). The crystals are coupled to two Hamamatsu
R1548 dual photomultiplier tubes ( PMTs ). The segmented BOO detector block is
currently being employed in a number of the commercially available positron
tomographs.
While the above mentioned block detector has significant advantages over
discrete detectors, better energy resolution, sensitivity and axial resolution, it also has
disadvantages such as: inter-detector scattering, and variations in sensitivity and energy
and spatial resolution across the face of the detector block.
In this study the variation in the efficiency, energy resolution, position of the
full energy photopeaks, spatial resolution and the amount of inter-crystal scatter
fraction for the individual detector crystals across the face of the detector block were
investigated. The factors contributing to these variations were identified and
suggestions for reducing their effects were made. For example, the inter-detector
scattering and the light sharing scheme, employed in the detector block for the
identification of the crystal of interaction, were found to be the cause of
mispositioning the events.
Two novel techniques which showed several advantages over the techniques,
employed by other groups, for evaluating the amount and distribution of inter-crystal
scattered events were also established. The amount of inter-crystal scatter fraction for
one of the central detector crystals, utilising both of the above mentioned techniques,
was obtained and found to be almost 36%. The spatial distribution of such scattered
events had a FWHM of 1.4 mm which its convolution with the so called ideal LSF
of the crystal deteriorated its FWHM by 0.5 mm.
It was shown that the energy distribution of the majority of the inter-detector
scattering events was around the 511 ke V peak and these scattering events were
detected within the full energy photopeak of the true events. Hence it is not possible
to reject or discard these events by means of setting a higher energy threshold. This
fact, in conjunction with the contribution of these events in worsening the spatial
resolution of the PET systems utilizing crystals of small dimensions, demands special
consideration to be given to the problem of inter-crystal scattering when further
reducing the size of the crystals. Therefore, it is recommended that prior to
incorporating detector blocks, which are cut into crystals of small dimensions, in PET
systems, the amount and the distribution of the scattered events and their contribution
in deteriorating the spatial resolution should be investigated by employing the
techniques established in this work.